http: protect against response splitting attacks

This patch is a back-port of 3c293ba.
Closes #4696
blog: v0.9.9
blog: v0.9.9 is unstable, not stable
test: disable simple/test-dgram-send-error

It's not a good citizen, it spams random IP addresses with UDP packets.

Fixes #4730.
Fix crypto.hmac behavior with empty keys.

node 0.9.6 introduced Buffer changes that cause the key argument of
Hmac::HmacInit (used in crypto.createHmac) to be NULL when the key is
empty. This argument is passed to OpenSSL's HMAC_Init, which does not
like NULL keys.

This change works around the issue by passing an empty string to
HMAC_Init when the key is empty, and adds crypto.createHmac tests for
the edge cases of empty keys and values.
test: fix tests after ECONNRESET patch
bench: make io.c file path configurable
blog: Peer Dependencies article

Thanks, @domenic
blog: Forgot slug on peer-dependencies article
net: Respect the 'readable' flag on sockets

Previously, we were only destroying sockets on end if their readable
side had already been ended.  This causes a problem for non-readable
streams, since we don't expect to ever see an 'end' event from those.

Treat the lack of a 'readable' flag the same as if it was an ended
readable stream.

Fix #4751
buffer: accept negative indices in Buffer#slice()

A negative start or end parameter now indexes from the end of the
buffer. More in line with String#slice() and ArrayBuffer#slice().
udp_wrap: don't use namespace imports in header

It's not really harmful but it's inconsistent with the other header
files.
udp_wrap: use proper imports in source file
stream_wrap, udp_wrap: add read-only fd property

Expose the file descriptor as a read-only property on the internal
handle objects. Intended for debugging purposes, not part of the API
proper. The property is always null on Windows.

Fixes #4754.
doc: add prompt to fix repl_test.js example

Running repl.start without the prompt set produces this error:

repl.js:95
    throw new Error('An options Object, or a prompt String are required');
          ^
Error: An options Object, or a prompt String are required
    at new REPLServer (repl.js:95:11)
    at Object.exports.start (repl.js:321:14)
    at Object.<anonymous> (/Users/dan/Dropbox/Documents/dev/nextgen/repl_test.js:5:6)
    at Module._compile (module.js:449:26)
    at Object.Module._extensions..js (module.js:467:10)
    at Module.load (module.js:356:32)
    at Function.Module._load (module.js:312:12)
    at Module.runMain (module.js:492:10)
    at process.startup.processNextTick.process._tickCallback (node.js:244:9)
doc: add tools/ dir to CONTRIBUTING.md verboten list
doc: clarify child_process.exec() stdio option

It only works for stdin, not stdout/stderr, for obvious reasons.
http: Raise hangup error on destroyed socket write

Prior to v0.10, Node ignored ECONNRESET errors in many situations.
There *are* valid cases in which ECONNRESET should be ignored as a
normal part of the TCP dance, but in many others, it's a very relevant
signal that must be heeded with care.

Exacerbating this problem, if the OutgoingMessage does not have a
req.connection._handle, it assumes that it is in the process of
connecting, and thus buffers writes up in an array.

The problem happens when you reuse a socket between two requests, and it
is destroyed abruptly in between them.  The writes will be buffered,
because the socket has no handle, but it's not ever going to GET a
handle, because it's not connecting, it's destroyed.

The proper fix is to treat ECONNRESET correctly.  However, this is a
behavior/semantics change, and cannot land in a stable branch.

Fix #4775
http: Do not let Agent hand out destroyed sockets

Fix #4373
npm: Upgrade to v1.2.11